1. Field of the Invention
This invention relates to a two-phase hydrogen permeation membrane utilizing the combination of a perovskite-type oxide and palladium metal and a process for separating hydrogen from a hydrogen-containing gas by means of the membrane.
2. Prior Art
Compressed natural gas is an economically competitive, widely distributed energy and chemical resource. This natural gas is converted to hydrogen more easily and efficiently than are liquid hydrocarbons and is less expensive per mole of hydrogen produced than any other fuel. Systems for producing hydrogen from natural gas could be enhanced through the use of thermally efficient, compact, catalytic membrane reactors.
Advances in membrane reactor technology allow economic production of high purity hydrogen from natural gas by coupling steam reforming and hydrogen transport in one step. Removal of product hydrogen continuously through the membrane shifts the equilibrium toward increased hydrogen production. Although palladium metal alloy membranes have been available for several decades, they are expensive and require large areas for adequate fluxes in commercial applications.
Palladium metal alloy membranes are being used for high purity hydrogen separation. Partial oxidation of natural gas has also been studied using catalyst and oxide ion conducting membrane technology. Currently, the combination of these two steps into a single step is not available.
Electrocatalytic conversion of methane to higher hydrocarbons and to synthesis gas has been reported in the literature and both approaches use solid, oxygen-ion conducting ceramics and involve partial oxidation. At high conversions, partial oxidation runs the risk of producing undesirable deep oxidation products (CO.sub.2 and H.sub.2 O), thus limiting hydrogen yields.
Recently, a series of perovskite-type oxides (e.g. BaCe.sub.1-x M.sub.x O.sub.3, where M is a metal dopant) have been shown to have a high proton conductivity at elevated temperatures.
These perovskite-type oxides have been shown to have a high proton conductivity and elevated temperatures, e.g. a conductivity of 10.sup.-2.OMEGA..sup.-1 cm.sup.1 at 600.degree. C.
This ionic conductivity is comparable to that observed for oxygen-ion conduction in La.sub.1-y Sr.sub.y Co.sub.1-x M.sub.x O.sub.3 perovskite-type oxides. La.sub.1-y Sr.sub.y Co.sub.1-x M.sub.x O.sub.3 are mixed conductors in that they conduct both oxygen ions and electrons, and have they received considerable attention for application as oxygen permeation membranes. Because of their significant electronic conductivity, they have an internal electrical short and O.sub.2 will selectively permeate through the material under a differential oxygen partial pressure (P.sub.O2). The potential permeation flux rates of these materials are extremely high. For example, calculations show O.sub.2 flux rates through a 50-.mu.m--thick membrane of La.sub.0.6 Sr.sub.0.4 Co.sub.0.8 Cu.sub.0.2 O.sub.3 at 600.degree. C. to be 22400 L (STP) h.sup.-1. m.sup.-2 of membrane surface area under a 0.21 atm P.sub.O2 gradient.
BaCe.sub.1-x M.sub.x O.sub.3 -type protonic conductors have sufficient ionic conductivity to obtain comparable flux rates. However, they have insufficient electronic conductivity. If comparable electronic conduction could be obtained with the BaCe.sub.1-x M .sub.x O.sub.3 -type protonic conductors, they would be excellent H.sub.2 permeation membrane materials, equivalent to palladium alloy films.
A second potential application of solid-state high temperature protonic electrolytes is the production of higher hydrocarbons such as C.sub.6 H.sub.6 and C.sub.7 H.sub.8 from CH.sub.4 : EQU 6 CH.sub.4 C.sub.6 H.sub.6 +9 H.sub.2
The decomposition and conversion of methane into benzene (C.sub.6 H.sub.6 and C.sub.7 H.sub.8) is thermodynamically favored at moderate temperatures (500.degree. C.) and moderate pressures (1 to 10 atm) when hydrogen is continuously removed to low levels (&lt;0.05 atm). A suitable dehydrogenation catalyst with low coking tendency (Pt or Pd), combined with a small pore zeolite for hydrode-cyclization of C.sub.2+ intermediates (such as C.sub.2 H.sub.4), could give high yields of aromatics. Electrochemical pumping is essential to increase the rate of H.sub.2 removal, since little driving force for H diffusion exists with low H.sub.2 partial pressures on both sides of the membrane.
Electrocatalytic conversion of methane to higher hydrocarbons and to syn gas has been reported in the literature. Both of these approaches used solid, oxygen-ion conducting ceramics. Under these conditions, both approaches are partial oxidation routes At high conversions, partial oxidation runs the risk of producing undesirable, deep oxidation products (CO.sub.2 and H.sub.2 O), thus limiting H.sub.2 yield. A preferable route is to electrocatalytically abstract an H from CH.sub.4 by using a protonic conductor. The resulting CH.sub.3 fragments would form higher hydrocarbons in the reacting gas stream, and pure H.sub.2 would be produced on the other side of the membrane.
As examples of the state of the art, Langley et al. U.S. Pat. No. 3,413,777 describes a hydrogen permeation membrane comprising palladium particles dispersed in a non-conductive glaze on a porous ceramic support.
In Taniguchi et al. U.S. Pat. No. 5,387,330, ionic conductors are described which are perovskite-type oxides of the formula BaCe.sub.1-x M.sub.x O.sub.3-.alpha.. In this formula, M is a rare earth element such as Gd, Tb, Dy, Ho or Er. These oxides have been shown to have excellent proton conductivity together with oxide ion conductivity. However, they exhibit no significant electronic conduction.
Lessing, U.S. Pat. No. 5,496,655 describes catalytic interconnected plates for fuel cells. Among a variety of electrolyte compositions, BaCe.sub.0.9 Gd.sub.0.1 O.sub.3 may be used. This system is used as a fuel cell for reformation of hydrocarbon fluids into hydrogen, CO and CO.sub.2.
Nishihara et al. U.S. Pat. No. 5,604,048 describes a tubular-type fuel cell containing an electro conductive ceramic. Perovskite-type composite oxides are used in the production of these ceramics.
In Shen et al. U.S. Pat. No. 5,616,223, mixed oxygen-ion and electronic conducting composite materials are described which include a CeO.sub.2 -based oxygen ion conductor material mixed with palladium or a palladium alloy as an electronic conductor metal phase.
Wallin, U.S. Pat. No. 5,670,270 relates to an electro chemical device with a solid state electrolyte membrane. Suitable ionically-conductive materials include gadolinium-doped ceria.
It is an object of the present invention to provide solid conductor materials having both good proton and electron conductivity.
It is a further object to provide an improved process for separating hydrogen from a hydrogen-containing gas.
It is still a further object to provide an improved process for converting methane to hydrogen and higher hydrocarbons.